1. Field of the Invention
The present invention relates to a robot device having a control unit to control a pair of actuators for rotational driving of a link, a robot control method, a program, a recording medium, and a control device.
2. Description of the Related Art
The ability for end effectors to flexibly come into contact with objects is becoming important in control methods of manipulators. Application of this to industrial robots could realize cooperative work between humans and robots, facilitating work of fitting parts to each other by controlling the direction of flexibility of end effectors, and so forth.
Also, applying this to legged locomotive robots could alleviate shock to the torso by coming into contact with the ground softly, and enable smooth ambulation over unsmooth terrain by absorbing difference in height.
In order to realize control of flexibility of the end effectors, impedance control where force sensors are worn on the end effectors, control using artificial muscle actuators, and so forth, is being performed. It is known that while human muscles are actuators, they are also control mechanisms with variable viscoelasticity. Rubber pneumatic artificial muscle actuators, of which the McKibben type artificial muscle actuator is representative, are similar to human muscles with regard to their viscoelastic properties. Now, controlling the softness of the artificial muscle actuators included in a manipulator enables contact with an object with optional end effector flexibility.
“Mechanical Properties of Robot Arm Operated with Muscle Coordinate System Consisted of Bi-articular Muscles and Mono-articular Muscles”, Toru OSHIMA, Tomohiko FUJIKAWA, and Minayori KUMAMOTO, Journal of The Japan Society for Precision Engineering, Vol. 66, No. 1 pp. 141-146 (hereinafter “Oshima et al”) proposes a “three-pair six-muscle model” manipulator having, in addition to artificial muscle actuators which drive a first link and a second link, a bi-articular simultaneous driving actuator to drive the first link and second link at the same time. Oshima et al have verified that by making the elasticity of the artificial muscle actuators to be equal, in a case where external force is applied to the end effectors along a line connecting a first joint and an end effector, there is exhibited a feature in stiffness characteristics in that the eternal force direction of the end effector and the direction of motion agree. McKibben actuators are difficult to use in motion control, due to the following reasons. The viscoelastic properties of McKibben actuators are nonlinear, and also control has to be applied to an antagonistic arrangement.
On the other hand, Japanese Patent Laid-Open No. 2012-86354 discloses deriving a simple feedback control system using saturation of control input, which controls joint angle at the same time as with stiffness control of the end effectors.
The dynamics of manipulator systems changes depending on the mass of the end effectors. A control system which does not take this change into consideration will exhibit degradation in capabilities to control the joint angle, due to grasping a payload at the end effectors, and so forth. Control based on passivity is known as a control technique to maintain control capabilities and stability in a case where change in dynamics has occurred in a manipulator system.
However, the manipulator control disclosed in Japanese Patent Laid-Open No. 2012-86354 includes an integrator in the feedback system, to compensate for joint torque according to elastic elements of the actuator. Accordingly, control based on passivity such as Proportional Derivative (PD) control is not readily applied, and there is a problem in that change in mass at the end effectors due to grasping a payload or the like causes degradation in capabilities of tracking control of joint angle.